Look at those two cases:

http://en.wikipedia.beta.wmflabs.org/wiki/Topic:Sl18xph3p9den4ob (deleted topic)
mw.flow.data.blocks.topic has no 'roots'.

http://en.wikipedia.beta.wmflabs.org/wiki/Topic:Sl18xaj7bextx9g8 (non deleted topic)
mw.flow.data.blocks.topic has 'roots'

The first one returned without permission to view; as a result, the topic is not rendered (and the DM should follow suit.)

This is probably true to more than just this case. Right now, the DM skips boards if .roots is undefined (which is the case above) but that is probably a bad thing to count on. We should have a proper way of validating whether the content we are on is viewable at all before we even attempt to send it to the data model and render it.